The present invention relates generally to nondestructive testing systems and methods using ultrasonics, and more particularly to improved system and method for nondestructive testing of composite laminates.
Composite materials, particularly fibrous composite laminates are of substantial interest to the aerospace industry for aircraft structural components by reason of high strength-to-weight ratios which characterize these materials. In the manufacture and use of composite materials, any of numerous performance limiting defects may arise within a composite structure, including delaminations, porosity, voids, ply gaps and overlaps, resin-rich and resin-lean regions, fiber misalignment, and cracks. Certain defects not detected in the laminate structure as manufactured may arise in the material during use. Nondestructive testing (NDT) methods in use heretofore for detection of the defects include x-ray radiography and ultrasonics in various modes such as pulse-echo and through-transmission. However, the inhomogeneous anisotropic structure which characterizes fibrous composites severely limits the suitability of conventional NDT methods for these materials.
According to the teachings of the cross reference, leaky Lamb waves may be used to detect and characterize defects in fibrous composite laminates, which defects may not be detectable using conventional NDT techniques. Leaky Lamb waves are excited in water-coupled composite laminate plates by ultrasonic isonification at appropriate angles and frequencies using a two transducer, pitch-catch geometry. The resulting leaky Lamb wave modes have minima which are characteristic of defects extant in the laminate including plate thickness, fiber volume fraction and resin content, variations of which within a few percent may not be significant or objectionable. The teachings and background material disclosed in the cross reference are incorporated here by reference.
The invention substantially solves certain shortcomings of existing NDT techniques and is a significant improvement over the teachings of the cross reference with respect to defect discrimination particularly in detection of delaminations and porosity in laminates with small local thickness variations. According to the invention, periodicity of leaky Lamb wave spectra, instead of single mode minima, provides a basis for the nondestructive evaluation of the laminate. The invention permits suppression of signals caused by changes in leaky Lamb wave mode structure resulting from small variations in laminate thickness, fiber volume fraction or resin content, but retains sensitivity to defects of interest, such as delaminations and porosity. In the practice of the invention, the frequency of a radio frequency (RF) tone burst is modulated over a range of from about 50 to 150% of the transducer center frequency at a rate of about 2 to 20 Hz. A frequency modulated tone burst is a gated radio-frequency signal where the radio-frequency is essentially constant within each individual burst, but where the radio-frequency changes slowly from burst to burst in a stepwise and periodic fashion. The modulation, after sonic interaction with the laminate, results in a time-domain signal, which varies nearly periodically at the output of a gated integrating amplifier. This output signal may in turn be filtered or processed in other manner to extract specific defect-related information, to detect sound material, and/or to suppress effects on the signal of small variations in test article thickness or fiber, volume fraction. Defects such as delaminations or porosity will change or disrupt the periodicity of mode excitation across the modulation band in a way that the effective repetition rate of the envelope-detected video signal, after interaction with the laminate, changes significantly in the presence of these flaws. Further signal processing, described below, allows the method to distinguish among flaws of various type and location in the laminate layup, but to ignore signal response from plate thickness changes at any preselected level of suppression.
It is therefore a principal object of the invention to provide an improved nondestructive evaluation system and method.
It is a further object of the invention to provide system and method for nondestructively testing composite materials.
It is another object of the invention to provide nondestructive testing system and method utilizing leaky Lamb waves.
It is another object of the invention to provide nondestructive system and method utilizing leaky Lamb waves and signal coding and decoding for distinguishing defects from small thickness variations in laminate material.
These and other objects of this invention will become apparent as the detailed description of representative embodiments proceeds.